1. Field of The Invention
The present invention relates to an MMIC (Monolithic Microwave Integrated Circuit), and more particularly to a multilayer transmission line using a ground metal with a slit, and a hybrid using the transmission line, in which signal lines are formed on dielectric layers provided on both sides of the ground metal with the slit.
In this specification, dielectric refers to not only plastic such as polymer based dielectric material, but also ceramics and semiconductors.
2. Description of Related Art
An MMIC of such a structure is known that a ground metal with a slit is sandwiched by two dielectric layers, and transmission lines are formed on the outer surfaces of the dielectric layers. For example, Toshiaki Tanaka, et al. "Slot-Coupled Directional Couplers Between Double-Sided Substrate Microstrip Lines and Their Applications", IEEE Transactions on Microwave Theory and Techniques, Vol. 36, No. 12, December 1988 discloses directional couplers with such a structure that a ground plane is sandwiched between two substrates with microstrip lines on respective outer substrate surfaces. U.S. Pat. No. 5,303,419 entitled to Ittipiboon et al. discloses magic-tees with a similar structure.
In these microwave hybrids, the slots are formed in the ground metals for providing coupling between the upper and lower signal lines. In other words, the signal lines function as coupled lines which are coupled with each other via the slots. This makes it possible to provide miniature, high performance hybrids.
These slots formed on the ground metals, on the other hand, have a function to reduce the capacitance between the upper and lower signal lines. The present invention, taking account of this, develops novel, miniature, inexpensive transmission lines and hybrids.